We propose to use the spontaneously hypertensive rat in order to study the evolution of left ventricular hypertrophy and the effect of blood pressure lowering drugs in preventing or causing regression of this hypertrophy. The progression of hypertrophy will be described quantitatively (a) by new morphometric techniques for measuring the ultrastructural composition, dimensions, and nexal interconnections of myocardial cells; (b) by microchemical methods for measuring myofibrillar and mitochondrial content; and (c) by new and more precise techniques for determining the "in vivo" intracellular ion and water contents and concentrations, with special emphasis on cellular sodium, chloride, and calcium. The project is designed both as a contribution to the physiology and pharmacology of hypertensive heart disease and as a demonstration of how to apply morphometric and allied techniques to study the growth of membranes and other ultrastructures in heart muscle cells.